1. Field of the Invention
The present invention relates to a particle beam therapy system capable of highly accurate therapeutic irradiation. More particularly, the invention relates to a particle beam therapy system that uses advanced irradiation technology such as the scanning method capable of highly precise therapeutic irradiation in conformity to a complex target shape.
2. Description of the Related Art
Against the background of an aging society in recent years, attention has been drawn to radiation therapies which, as cancer treatment, are minimally invasive to the body and permit a high quality of life to be maintained after the treatment. One of the most promising of these therapies is the particle beam therapy system that offers good dose concentration on the tumor using a charged particle beam of protons, carbon particles, etc., accelerated by an accelerator. The particle beam therapy system is typically composed of an accelerator such as a synchrotron that accelerates a beam from an ion source close to light speed, a beam transportation system that transports the extracted beam from the accelerator, and an irradiation device that irradiates the patient with the beam in conformity to the position and shape of the tumor.
In applying the beam in conformity to the tumor's shape with the irradiation device of the particle beam therapy system, the scatterer irradiation method is adopted whereby the beam diameter is enlarged using a scatterer before being trimmed to shape by a collimator trimming the beam diameter periphery, or the scanning method is used which involves a small-diameter beam from an accelerator being deflected by magnets to scan the tumor in conformity to its shape. Coming into the mainstream in recent years is the scanning method capable of highly accurate therapeutic irradiation in conformity to a complex target shape. As with the conventional scatterer irradiation method, the scanning method is required to provide a standard nominal value of 2 Gy/min in dose rate for every 1 L of irradiation volume with a view to shortening therapeutic irradiation time.
The scanning method involves dividing a three-dimensional tumor shape into a plurality of layers in the depth direction and further diving each of the layers two-dimensionally to set up a plurality of irradiation spots. In the depth direction, each of the layers is irradiated selectively with a beam at varying energy levels. In each layer, the irradiation beam is moved two-dimensionally for scanning so as to give a predetermined dose to each of the irradiation spots. A method of continuously activating the irradiation beam during movement between irradiation spots is called raster scanning. On the other hand, a method of deactivating the irradiation beam during such movement is called spot scanning. Where a synchrotron is used as the accelerator with any of these scanning methods, a charged particle beam is accelerated to an energy level corresponding to each of the layers in the depth direction of the tumor before the beam is extracted to the beam transportation system. By way of the beam transportation system, the charged particle beam is transported to the irradiation device which in turn applies the transported beam selectively to the layers of the tumor in question.
Conventional methods for operating the synchrotron typically involve accelerating a charged particle beam injected from a pre-accelerator up to a predetermined energy level, extracting the charged particle beam at the energy level in effect upon completion of acceleration, and decelerating the residual beam upon completion of extraction down to the energy level in effect upon beam injection before discarding the beam. That is, the conventional synchrotron simply repeats an operating cycle of injection, acceleration, extraction and deceleration. During the extraction phase of one operating cycle, only the charged particle beam at a single energy level can be extracted. Thus in cases where the irradiation beam at a plurality of energy levels is needed to address a plurality of layers of the tumor in the depth direction with the scanning method, it is necessary to again carry out deceleration, injection and acceleration every time the energy level is changed even if there remains a sufficient orbiting beam in the synchrotron. This poses the problem about prolonged therapeutic irradiation time because of reduced dose rates.
As one solution to the above problem, Japanese Patent No. 4873563 discloses an operation method involving a synchrotron successively accelerating or decelerating the orbiting beam during the extraction phase so as to extract a charged particle beam at a plurality of energy levels. The synchrotron operation method described by the above-cited patent indeed shortens the time required to change energy levels so as to improve dose rates thereby shortening therapeutic irradiation time. However, the disclosed method is not necessarily sufficient to attain the required standard nominal value of 2 Gy/min in dose rate for every 1 L of irradiation volume.
The results of experiments with a synchrotron for carbon-beam treatment realized by implementing the operation method described in the above-cited patent are disclosed in “Multiple-energy operation with extended flattops at HIMAC,” Nuclear Instruments and Methods in Physics Research A624 (2010) 33-38. The latter publication describes an operation method whereby the size of a stability limit is enlarged using quadrupole electromagnet prior to acceleration or deceleration of a charged particle beam so as to suppress the extraction of an unnecessary charged particle beam from the synchrotron at the time of acceleration or deceleration for energy level change, the stability limit being again reduced to the original size upon completion of acceleration or deceleration. This method entails the problem about low speed response of the quadrupole electromagnet because of their large inductance values, which prolongs the time required to change energy levels. Although there exists a method involving a synchrotron being separately equipped with dedicated small-inductance quadrupole electromagnet permitting high-speed response, this method can lead to the synchrotron getting larger in size and higher in cost. This is not a realistic option for the synchrotron for proton-beam treatment that must be small in size and low in cost.